In the early days of the Civil War (which pro-vs-anti-slavery war was anything but "civil"), Treasury Secretary Salmon P. Chase requested the director of the U.S. Mint in Philadelphia to prepare a motto for American coins.

'Dear Sir,' his reply was, 'No nation can be strong except in the strength of God, or safe except in His defense. The trust of our people in God should be declared on our national coins.'

After a few options were considered, Chase decided on: IN GOD WE TRUST.
Congress passed the concordant legislation, and that motto first appeared on the 1864 two-cent coin.

IN GOD WE TRUST has been in continuous use on the penny since 1909, and on the dime coin since 1916.

In fact, since 1938, all United States coins have borne that inscription.

[ When that motto was left off a new double-eagle gold coin in 1907, the public's outcry led Congress to order it restored. ]

In 1956, the Congress and President Eisenhower agreed to declare IN GOD WE TRUST "the national motto of the United States."

That motto then began to appear on our paper money also.